1. Field of the Invention
The present invention relates to feedforward power amplifiers, and more specifically to a method and circuit for conducting adaptive linearization of power amplifiers.
2. Brief Description of the Prior Art
RF power amplifiers are used in a wide variety of communications and other electronic applications. These amplifiers are made of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the stage gain. Ideally, the input-to-output transfer of each stage is linear; that is, a perfect replica of the input signal increased in amplitude appears at the amplifier output. In reality, however, all RF amplifiers have a degree of non-linearity in their transfer characteristics. This non-linearity results in a distortion of the output signal so that it is no longer a perfect replica of the input. This distortion produces spurious signal components known as intermodulation products. Intermodulation products are undesirable because they cause interference, cross-talk and other deleterious effects on the performance of a system employing RF power amplifiers. Accordingly, the prior art reflects various methods and devices designed to reduce the distortion produced by RF power amplification.
Three methods have commonly been used in the prior art to reduce distortion: predistortion, feedback and feedforward. The main advantages of the feedforward approach over the two others methods are an unconditional stability over large frequency bandwidths without inherent frequency limitations, and a high degree of distortion cancellation irrespective of the order of the intermodulation products.
With the rapid development of satellites, mobiles, cellular radios and PCS communication systems, a common broadband power amplifier for a multi-channel usage is eagerly desired. This demand is stimulating continuous investigation of the feedforward linearization technique.